1. Field of the Invention
The present invention relates to a corrected mask pattern verification apparatus and a corrected mask pattern verification method for verifying that a pattern of a corrected mask has been properly corrected. More specifically, the present invention relates to a corrected mask pattern verification apparatus and a corrected mask pattern verification method for verifying that correction of a pattern of an exposure mask in consideration of the influence of the optical proximity effect, generated when exposing the exposure mask to light, has been properly performed.
2. Description of the Related Art
In a production process of a semiconductor integrated circuit, photolithography is performed a plurality of times. In a photolithography step, a photomask is formed as an exposure mask. The photomask has a pattern corresponding to a layout of the semiconductor integrated circuit represented by design data for the semiconductor integrated circuit. In the photolithography step, the photomask is exposed to light on a wafer so as to transfer the pattern of the layout onto the wafer.
Such a photomask generally has the same pattern as that of a light shielding portion of a photoresist layer formed on a quartz substrate. The pattern of the photomask is formed based on the pattern of the light shielding portion and a light transmitting portion of the photoresist layer.
Recently, as semiconductor integrated circuits are increasingly reduced in size, the patterns of the layout of the semiconductor integrated circuits are also increasingly becoming smaller. In accordance with this, the patterns of the photomasks are also becoming smaller.
When a pattern of a photomask is extremely small, interference caused by diffraction of light influences the pattern transferred onto the wafer. As a result, the pattern transferred onto the wafer may be significantly different from the pattern represented by the design data. In this case, malfunctions may occur, which lowers the production yield of the semiconductor integrated circuits.
Hereinafter, the influence of interference on the pattern transferred onto the wafer will be described.
FIG. 18 shows a corner of a pre-correction (original) mask 51. FIG. 19 shows a corner of a pattern 52 transferred onto a wafer by exposing the pre-correction mask 51 shown in FIG. 18 to light. The corner of the pattern 52 is rounded due to interference, and loses a corner portion 52a. 
In order to suppress the influence of interference of light on the pattern transferred onto a wafer, a technology referred to as optical proximity effect correction (OPC) has been developed. The processing used by this technology (OPC processing) is performed as follows. The influence of interference of light on the pattern transferred onto the wafer is evaluated by optical simulation or using test patterns transferred onto the wafer by exposure to light. Based on the evaluation results, the pattern of the photomask is corrected such that the pattern transferred onto the wafer reproduces the pattern represented by the design data. The pattern transferred onto the wafer by exposing the photomask having a corrected pattern reproduces the pattern represented by the design data with higher fidelity than a photomask having a non-corrected pattern.
Hereinafter, the OPC processing will be described.
FIG. 20 shows a corrected mask 53 having a pattern corrected by the OPC processing. The corrected mask 53 has a projection 53a at the corner of the original mask 51 (FIG. 18).
FIG. 21 shows a corner of a pattern 54 transferred onto a wafer by exposing the corrected mask 53 shown in FIG. 20 to light. The corner of the pattern 54 is rounded by interference, and loses a corner portion 54a. However, the pattern 54 obtained using the corrected mask 53 reproduces the design data with higher fidelity than the pattern 52 (FIG. 19) obtained using the pre-corrected mask 51.
There are two principal methods for correcting a pattern by the OPC processing for producing a semiconductor integrated circuit. According to method 1, the degree of correction is determined based on an optical simulation and the pattern is corrected (simulation-based OPC). According to method 2, pattern correction is performed based on a specific rule for each pattern (rule-based OPC).
Generally, method 2 is inferior to, but consumes less time than, method 1. In general, method 1 or method 2 is used in accordance with the required precision of the pattern.
The OPC processing corrects the entire pattern of the photomask used for producing a semiconductor integrated circuit. More specifically, the photomask is divided into small regions (having sides of several micrometers to several hundred micrometers), and each of the small divided regions is corrected. Each side of each pattern is required to be sufficiently longer than the wavelength of light, but should also be appropriately short such that the amount of data is sufficiently small to be processed at one time regardless of the performance of the computer used.
The border regions between adjacent small regions may not be properly corrected for the following three reasons: (1) the adjacent small regions are corrected based on different standards; (2) the standards for correction depend on how the border line is set; and (3) incorrect processing is performed by a correction program or a computer.
When the pattern of the photomask cannot be properly corrected, the pattern represented by the design data cannot be reproduced with high fidelity. As a result, pseudo defects are generated in the pattern of the photomask during the mask formation step, which lowers the production yield, and increases the cost of the semiconductor integrated circuits.
In order to ensure that a photomask having a properly corrected pattern is produced, methods for verifying that proper correction has been performed have been proposed.
Japanese Laid-Open Publication No. 11-174659 discloses a method for verifying that proper correction has been performed. The method disclosed in Japanese Laid-Open Publication No. 11-174659 includes the step of determining whether or not a corrected mask pattern is encompassed in a mask pattern obtained by enlarging an original mask, and the step of determining whether or not a mask pattern obtained by reducing the original mask is encompassed in the corrected mask pattern.
FIG. 22 shows the corrected mask pattern verification method disclosed in Japanese Laid-Open Publication No. 11-174659. This method will be described with reference to FIG. 22.
Original mask pattern data (design mask pattern data) 11 and corrected mask pattern data (post-correction mask pattern data) 12 are input to a corrected mask pattern verification apparatus. The original mask pattern data 11 represents the pattern of an original mask. The corrected mask pattern data 12 represents the pattern of a corrected mask.
The original mask pattern data 11 is processed by graphic reduction operation 13, thereby generating reduced mask pattern data, and also processed by graphic enlargement operation 14, thereby generating enlarged mask pattern data.
The generated reduced mask pattern data and the corrected mask pattern data 12 are processed by graphic differential operation 15, thereby generating first differential mask pattern data. The first differential mask pattern data indicates how much of a reduced mask pattern represented by the reduced mask pattern data is encompassed in a corrected mask pattern represented by the corrected mask pattern data 12.
Based on the first differential mask pattern data, determination 17 is performed, thereby generating first determination data which indicates whether or not the reduced mask pattern is encompassed in the corrected mask pattern.
The generated enlarged mask pattern data and the corrected mask pattern data 12 are processed by graphic differential operation 16, thereby generating second differential mask pattern data. The second differential mask pattern data indicates how much of the corrected mask pattern represented by the corrected mask pattern data 12 is encompassed in the enlarged mask pattern represented by the enlarged mask pattern data.
Based on the second differential mask pattern data, determination 18 is performed, thereby generating second determination data which indicates whether or not the corrected mask pattern is encompassed in the enlarged mask pattern.
Whether or not the corrected mask has been properly corrected is determined based on the first determination data and the second determination data. When the first determination data indicates that “the reduced mask pattern is encompassed in the corrected mask pattern” and the second determination data indicates that “the corrected mask pattern is encompassed in the enlarged mask pattern”, the corrected mask is determined to have been properly corrected.
FIG. 23 shows an exemplary original pattern 11a represented by the original mask pattern data 11. The original pattern 11a is L-shaped as shown in FIG. 23.
FIG. 24 shows an exemplary enlarged mask pattern 14a represented by the enlarged mask pattern data. The enlarged mask pattern 14a is also L-shaped. Each of the sides of the enlarged mask pattern 14a is uniformly longer than each corresponding side of the original mask pattern 11a. 
According to the method disclosed in Japanese Laid-Open Publication No. 11-174659, abnormal correction may not be detected. The reason is that even when the first determination data indicates that “the reduced mask pattern is encompassed in the corrected mask pattern” and the second determination data indicates that “the corrected mask pattern is encompassed in the enlarged mask pattern”, the corrected mask may have been abnormally corrected.
Such a case will be described below.
FIG. 25 shows an exemplary corrected mask pattern 55 having an unnecessary pattern 55a. 
FIG. 26 shows an exemplary corrected mask pattern 56 lacking a necessary pattern 56a. 
The corrected mask pattern 55 and the corrected mask pattern 56, neither of which is a properly corrected mask pattern, are both determined as a properly corrected mask pattern by the method disclosed in Japanese Laid-Open Publication No. 11-174659. The reason is that the first determination data indicates that “the reduced mask pattern is encompassed in the corrected mask pattern” and the second determination data indicates that “the corrected mask pattern is encompassed in the enlarged mask pattern”. The size of the corrected mask pattern 55 and the size of the corrected mask pattern 56 are substantially the same as the size of the proper corrected mask pattern.
Japanese Laid-Open Publication No. 11-184064 discloses a corrected mask pattern verification method for verifying that the corrected mask pattern has been properly corrected by comparing an original mask pattern and a corrected mask pattern based on an optical simulation. This method, however, increases the cost of the semiconductor integrated circuits, since the operation steps such as Fourier transform steps performed by a computer extends the processing time.